1. Field of the Invention
The present invention relates to a flexible cable having excellent dielectric resistance against static electricity.
2. Description of the Prior Art
A flexible printed circuit board in recent years has been profitably employed, for example as a circuit board or as wiring parts, in various fields as an important part for use in mounting thereon many electronic circuits which are central to computer applications. Such a flexible printed circuit board has a multilayer structure adapted to directly mount electronic parts thereon, is miniaturized and lightweight, and its flexible structure facilitates use within a limited space for higher density mounting. Employed as wiring parts, the flexible printed board is used to wire together individual connections in a wire harness so as to be useful for the elimination or reduction of labor as well as prevention of erroneous wiring. In addition, it is also in wide use for wiring various movable portions in view of the flexible and lightweight structure thereof.
Referring to FIGS. 1(a) and 1(b) illustrating a prior flexible cable structure, a flexible cable has a conducting part 1, and at each end thereof a terminal 2 which is an extension of the conducting part 1, and a lug 3 for attaching thereto setscrews in locking holes 4. The flexible cable includes a base 5 comprising a high polymer material such, for example, as polyester and polyimide. The base 5 is formed of a flame-retardant material with a thickness of several tens of micrometers. Designated at 6 are copper foil patterns forming the conducting part 1 and patterned by subjecting a copper foil bonded onto the base 5 to etching. A cover layer 7 formed by the same material as that of the base 5 is arranged to cover the copper foil patterns 6 on the base 5. Designated at 8 is an insulator part comprising the base 5 and the cover layer 7. Since the terminals 2 are intended to be brought into contact with a pattern of another part (not shown) a portion of the cover layer 7 is eliminated in the area of terminals 2. Moreover, the terminals 2 are, since they are subject to deterioration of contact resistance, subjected to further processing, for example a gold plating operation. The copper foil patterns 6 are positioned at intervals capable of ensuring dielectric strength with respect to adjoining pattern portions and are maintained a distance away from both sides of the insulator 8. The cover layer 7 and the copper foil patterns 6 respectively have thicknesses of about several tens of micrometers and are capable of bending. Accordingly, the flexible cable may be used for transmitting an electric signal to a movable mechanical member such as a printer carriage cable.
However, such a prior flexible cable suffers from certain drawbacks. Namely, when an object having static electricity of high voltage is brought near a flexible cable of the type described above, a high electric field is generated between the conductive part of the cable and the object, whereby any high polymer material such as polyester, etc., located therebetween is polarized. Therefore, the cover layer is subjected to dielectric breakdown and this causes electric discharges to be produced among different of the copper foil patterns. Thus, any electric circuit connected with the copper foil patterns is subject to damage.